How to spy on someone else’s phone?

To spy on someone else’s phone, one has to decide on what kind of information they want to know. Why? Because needs condition the picking of the right instruments. Thus, to spy on the phone’s location, you can locate it with its IMEI, Google’s Find My Device or Apple’s Find My iPhone, or within a concrete program of a mobile provider (Verizon, AT&T, T-Mobile).
If the subject of interest is calls or someone else’s online activities, then spyware is a must. It allows to track locations, SMS, Internet activities, installed apps, etc. Its biggest asset it the ability to run in the background and stay invisible. In due turn, it guaranteed that a user won’t be identified.